Parenteral administration of gram negative endotoxin elicits an ocular response in rabbits and rats that is clinically and pathologically similar to acute anterior uveitis in humans. Gram negative endotoxin has been postulated to play a role in the development of human uveitis. Indirect evidence of an association of bacterial products has been implicated in the development of HLA-B27 associated uveitis. However, a direct causal effect of endotoxin in human ocular inflammation has never been shown. Using fluorophotometry we have measured capillary permeability of fluorescein after administration of endotoxin to normal humans. No significant alterations in capillary permeability in the eye occurs. Thus, a dose of endotoxin capable of eliciting multiple arms of the acute phase response in humans (including increased intestinal permeability and lung permeability) spares the capillaries of the eye. This data suggest a considerably more complex pathogenesis of uveitis in humans than inferred from animal experiments.